1. Field of the Invention
This invention relates to a two-way spatial optical communication apparatus utilizing space as a transmission path to transmit and receive signals with a light beam as a medium, and particularly to an apparatus provided with an optical axis deviation correcting function.
2. Related Background Art
Recently, attention has been paid to a so-called spatial optical communication apparatus which effects information transmission by transmitting and receiving a modulated light beam through free space.
When light beam communication is to be effected in two ways, as shown in FIG. 13 of the accompanying drawings, a beam of light signal is sent from a transmitter and receiver 20 to the partner apparatus, but the partner apparatus must be confined within the light beam, and the partner apparatus receives that beam and receives the light signal, whereby communication can be accomplished.
A light beam, even after angular adjustment has once been effected, may have its angle varied by vibration, wind pressure, distortion by solar radiation, refraction by the temperature distribution of the atmosphere or the like and may deviate from the partner apparatus. Accordingly, means becomes necessary for detecting and adjusting the position of the beam during the position adjustment at the time of installation and during the operation of the apparatus.
As a method of detecting the position of a beam, a method of disposing a plurality of receiving lenses 21 around a transmitting lens 22 as shown in FIG. 14 of the accompanying drawings, and detecting the deviation of a received beam from the center by the distribution of quantities of light entering the respective receiving lenses is proposed in Japanese Laid-Open Patent Application No. 1-226228. Also, in a system wherein a receiving lens and a transmitting lens can be used in common, there is a case where a plurality of lenses exclusively for detecting the position of a beam are disposed around a main lens. In this case, the form becomes similar to that shown in FIG. 14, and there is sometimes used jointly a system in which besides a main signal transmitted, position detecting auxiliary signals (also called pilot signals) are transmitted while being superposed one upon another.
The deviation of the beam at the receiving point is due to the deviation of the transmission angle of the partner apparatus and therefore, to correct the deviation of the beam, it is necessary to send that information back to the partner apparatus. Popular means therefor is to direct the entire apparatus by the use of a system in which signals indicative of the position of a beam are transmitted while being multiplexed upon a main signal or a system which utilizes other circuit such as telephone or radio.
On the other hand, Japanese Laid-Open Patent Application No. 2-186311 discloses an apparatus in which a movable mirror is provided in the optical path of a light beam to thereby correct the deviation of the optical axis. This example of the prior art is shown in FIG. 12 of the accompanying drawings. According to this apparatus, provision is made of a mechanism 108 in which two mirrors are provided in the transmission optical path to thereby correct optical axes in two directions independently of each other, and a mechanism 107 in which two mirrors are further provided in the coaxial transmission and reception optical paths to thereby correct optical axes in two directions independently of each other, thereby correcting the angle.
In case of the former system, it is necessary to send the position information of the reception light beam back to the partner apparatus, and for that purpose, there becomes necessary on the side which transmits the position information a circuit for converting the position signal into a multiplexable form (digitalizing it, changing it into a serial signal, adding a synchronizing signal or the like thereto, and further modulating the same) and a circuit for multiplexing the position signal, and on the side which receives the position signal, there becomes necessary a circuit for effecting the converse processing. Therefore, the construction of the apparatus becomes complicated and costly. Also, the multiplexing of the position information signal leads to the problem that the transmission power for the main signal becomes correspondingly small. Also, where other circuit such as telephone is utilized to transmit the position signal, circuits for signal conversion, the control of transmission procedure, etc. are necessary on both sides, and this similarly leads to high costs and further, extra cost such as circuit fees will be required. Also, besides the problem in the transmission of the beam position information, beam position detecting means itself requires a number of optical systems as can be seen from FIG. 7 of the accompanying drawings, thus causing the bulkiness and high cost of the apparatus.
On the other hand, where use is made of optical axis deviation correcting means like the latter, at least four mirrors in total become necessary and a considerably long extension of the optical path becomes necessary, while there is the disadvantage that the cumulation of the deviation of the light beam by the change in the angles of the mirrors is great and this leads to the bulkiness of the apparatus or the inability to secure the range of corrected angle very widely. Also, when the optical axis of the received light is to be corrected, the optical axis of the transmitted light is changed at the same time, and this correction also becomes necessary. Accordingly, a complicated control signal processing circuit becomes necessary to effect the quick correction of the optical axes of the transmitted and received lights, and this leads to increased costs.
On the other hand, Japanese Laid-Open Patent Application No. 63-120534 discloses a technique of varying the directional angle of an emergent beam so that light may be reliably transmitted to the reception side apparatus. That is, as shown in FIG. 9 of the accompanying drawings, a laser source 103 is moved to thereby vary the directional angle.
Now, where in a prior-art apparatus as shown in FIG. 26 of the accompanying drawings, a laser diode is used as a light source 103, a cooling apparatus usually becomes necessary, and a light source mounting bed 104 must be moved with this cooling apparatus. In this case, there has been the disadvantage that a load is applied to a movable knob and it is difficult to make the light source 103 rectilinearly movable with good accuracy and the light source is liable to cause an oscillated state and even the direction of emergence of the center of light emission of the transmitted beam is liable to change.
On the other hand, it is also conceivable to change the directional angle of the transmitted beam by moving a lens unit 77 having positive power which constitutes a beam expander as shown in FIG. 18 of the accompanying drawings along the optical axis, but since the optical path from a reference state is changed on the transmission side and the reception side, the amount of movement of the focus of the reception light on the light receiving element side is amplified. Accordingly, the amount of variation in the spot diameter of the reception light beam on the light receiving surface tends to become great, and when the communication distance is a short distance, the amount of blur is great and the quantity of received light becomes deficient, and this has led to the disadvantage that the directional angle of the transmission beam cannot be sufficiently widened.